Animals
as diverse as birds, sea turtles, bats, and bees can sense the earth's magnetic
field and use it as a guide in navigation. A new study identifies the molecule
that may hold the key to this special ability. Véronique LaCapra brings us this
story.

Movement
in the Earth's iron core produces a magnetic field around the planet. Humans
can use the magnetized needle of a compass to navigate. But some animals can
perceive this magnetic field directly to orient themselves to their
surroundings.

The
exact mechanism underlying this remarkable magneto-sensitivity has remained a
mystery.

To
investigate this phenomenon, scientists needed a test species. "One of the
best animals to use is the fruit fly, because you can manipulate genetically
various aspects of the fruit fly's biology," says Dr. Steven Reppert, a neurobiologist at the University of Massachusetts Medical School.

To
confirm that fruit flies were able to sense a magnetic field, the researchers
developed an elegant experimental apparatus: a T-shaped maze. Picture the
letter "T" made out of hollow tubes, with a metal coil at the end of
each horizontal arm. By running an electric current through the coils, the
scientists could generate a magnetic field in either arm of the "T".

To
run the tests, the researchers put fruit flies into the maze, starting at the
bottom. When the flies reached the top of the T, Reppert says, "They could
either go to the left or to the right." One way took them toward the
magnetic field, the other, away from it.

Next
the scientists trained the flies by putting some sugar at the end of the
magnetized arm of the T-maze. Then they took the sugar away, and tested whether
the flies would still go towards the magnetic field, no matter which arm it was
in, expecting a sweet reward.

"What
we wanted to do was to see whether the flies would associate the food reward
with the magnetic field," explains Reppert. "And in fact the flies
did, and it gave us a much more prominent response of the flies to the magnetic
field."

Now
that Reppert and his colleagues were sure the flies could detect a magnetic
field, they wanted to figure out how they were doing it. Previous research had suggested that animals
may use special light receptors, called cryptochromes. "Cryptochromes are
proteins that function as blue light photoreceptors in the fruit fly."

In
ordinary light conditions, the cryptochromes in a fruit fly's eye and brain are
exposed to the full spectrum of light, from blue to green to red. The blue part
of the light spectrum activates the cryptochromes, causing them to undergo
specific chemical responses that the fruit flies need for their biological
clock to function.

To
test whether the cryptochromes also play a role in the fruit fly's ability to
sense magnetic fields, the researchers put a light filter over the T-maze.
"So the flies still could see red light and green light," Reppert
describes, "but the blue and ultraviolet shorter wavelengths were blocked.
And in that instance, the magneto-receptive response was totally gone."

Reppert
and his colleagues had shown that the flies needed blue light to detect a
magnetic field. But the researchers still had to make a definitive link between
magneto-sensitivity and cryptochromes.

To do
this they used fruit flies with genetic mutations that effectively disabled the
cryptochrome gene. "And then we could ask the question," says
Reppert, "since the gene is no longer working, what happens to the
magneto-sensitive response? The prediction was it would go away, and indeed it
did."

Migratory
birds, sea turtles, and other animals that use the earth's magnetic field to
navigate also have cryptochromes, albeit different ones from those found in
fruit flies.

Reppert
says that the next challenge will be to investigate whether the cryptochromes
of other species play a similar role to those of the fruit fly in sensing
magnetic fields. "We have the possibility in the fly to actually take any
animal's cryptochrome, to put it in as a trans-gene, and to ask the question in
the fly, can it function as a magneto-sensitive molecule, and we're very
excited about that potential."